The present invention relates generally to the field of lasers and in particular to methods and apparatus for configuring two-dimensional multi-wavelength laser source arrays into composite coaxial beams having higher laser power.
Laser systems that use multiple laser sources or multiple laser gain medium, are utilized in a variety of applications including cutting, machining, welding, material processing, laser pumping, fiber optic communications, free-space communications, illumination, imaging and numerous medical procedures. Many of these applications can be significantly benefitted with higher laser power. In support of achieving higher laser power, the input energy is typically increased. However, simply increasing the input energy may introduce additional thermal management considerations. For example, thermal conditions and heat load within the laser gain medium typically contribute to internal aberrations and corresponding beam quality reductions in the emitted radiation. Additionally, unaddressed internal heating may also lead to internal damage of the laser components themselves. In general, issues like these place practical limits on the achievable laser power for a given laser system design approach. In many cases the most cost effective method for further power scaling is achieved by combining the optical outputs from more than one laser or laser gain medium.
The ability to focus a laser beam into a small spot is generally characterized by its beam quality which is in part, a measure of its usefulness in a many applications. Ideally, laser power scaling through beam combining of multiple laser sources or multiple laser gain medium would be done in a manner that minimizes the reduction in the beam quality of the combined beam. When considered in combination, both laser power and laser beam quality contribute to what is typically termed beam brightness. When either or both laser beam power and laser beam quality are improved, the brightness of the laser beam is said to be improved. Beam brightness, being a measure of the combination of the power and focusability of a laser beam, is a fundamental measure of a laser beam's overall utility in many high power applications.
Historically, many methods have been used to advance the above objectives with varying degrees of success. These methods can be organized into three broad categories of design approaches, namely coherent, incoherent and polarization approaches. Methods characterized by coherent approaches have the ability to power scale significantly but require a high degree of mutual coherence between the laser sources. They generally employ real time beam phasing techniques between the laser sources or laser gain medium that are complex and costly to implement. Polarization approaches are simple to implement but, by themselves, do not scale beyond a factor of two, one for each available polarization. There are many approaches that do not use either mutual coherence or polarization to combine beams. These fall within the category of incoherent approaches. In general, incoherent approaches are easier to implement then coherent approaches.
One of the simplest incoherent approaches employs side-by-side beam combining whereby the laser sources or laser gain medium are arranged side-by-side, propagate nominally parallel to one another, are not overlapping or coaxial, and are not phased to each other. This incoherent side-by-side combined beam can be focused but does not produce an optimal focused spot for the given diameter of the side-by-side combined beam. In this case, the beam quality is said to be reduced. Incoherent approaches that both power scale and also maintain good beam quality generally employ specified and unique wavelengths for each laser source or laser gain medium as a fundamental aspect in the combining process. This technique is often called wavelength or spectral beam combining. Examples of components often employed in these systems are dispersive prisms, dialectic wavelength filters, volume Bragg gratings, and diffraction gratings. Naturally, this approach leads to combined beams having many wavelengths, or put another way, a large laser linewidth. In some laser applications, having a large laser linewidth may not be a desirable feature, but for many other laser applications, a large laser linewidth may be inconsequential and may even be advantageous in still other laser applications.
Within the prior art, two dispersive methods of wavelength beam combining employing angular dispersion have been disclosed that achieve the challenging task of combining two-dimensional laser emitter arrays while maintaining beam quality. In both of these dispersive methods, a plurality of wavelength dependent dispersive elements are used sequentially in two essentially orthogonal directions to combine the two-dimensional array of beams into a single composite beam. In the first method, the orthogonal dispersive elements include a first-order diffraction grating or prism and an Echelle diffraction grating operating in several high diffracting orders simultaneously. In a second method, the orthogonal dispersive elements include a first-order diffraction grating or prism and a first-order diffraction grating stack made up of a plurality of individual first-order diffraction gratings each having a different amount of angular dispersion. An example of a two-dimensional wavelength beam combining system that includes the use of high-order Echelle gratings is disclosed in U.S. Pat. No. 6,327,292 to Sanchez-Rubio et al. filed on Jun. 21, 1999. Two different examples of two-dimensional wavelength beam combining systems employing the use of a first-order diffraction grating stack are disclosed in U.S. Pat. No. 8,179,594 to Tidwell et al. filed on Jun. 30, 2008 and U.S. Patent Application No. US 2011/0222574 A1 to Chann et al. filed on Mar. 9, 2010.